Fast Breeder Reactor

A fast breeder reactor is a nuclear reactor that creates more fissile material than it destroys during operation. It uses surplus fast neutrons from fission to convert fertile material, which cannot readily sustain the chain reaction, into fissile fuel.

Breeding mechanism

The reactor core contains fissile fuel, commonly plutonium mixed with uranium. Fission releases neutrons: some sustain the chain reaction, while surplus neutrons are captured by fertile material placed in the core or a surrounding blanket.

• In the main uranium pathway, uranium-238 captures a neutron and undergoes radioactive decay through uranium-239 and neptunium-239 to form fissile plutonium-239.
• In a thorium fuel cycle, thorium-232 can similarly be converted through thorium-233 and protactinium-233 into fissile uranium-233.
• Breeding occurs when the breeding ratio, fissile material produced divided by fissile material consumed, is greater than one.

Why fast neutrons are used

The reactor does not use a moderator to slow neutrons. Its fast-neutron spectrum provides a favourable neutron economy, especially for plutonium fuel, leaving enough neutrons after sustaining fission to convert fertile nuclei.

• A coolant such as liquid sodium removes heat while slowing neutrons only slightly.
• Neutrons lost through leakage or non-productive absorption reduce the breeding ratio, so reactor geometry and materials are designed to conserve neutrons.

Fuel-cycle significance

The newly formed fissile material remains mixed with irradiated fuel and must be recovered through reprocessing and fabricated into fresh fuel. Breeding therefore extends uranium resources and can support conversion of thorium into uranium-233, but it still requires an initial fissile inventory and a closed fuel cycle.

• In India's three-stage nuclear power programme, fast breeder reactors form the second stage, using plutonium and uranium-238 to expand the fissile fuel base before larger thorium utilisation.